In an agricultural baler, material such as hay, straw, silage or similar crop material that previously has been cut, windrowed or swathed, is picked up from the field by a pick-up unit, fed into a duct and loaded into an elongated bale chamber. A plunger which reciprocates in the front portion of the baling chamber compresses the newly introduced material against the previously introduced material into a parallelepipedic package, and at the same time, gradually advances the package towards the outlet of the bale chamber. As the package reaches a predetermined length as determined by a metering device, a knotter device is actuated to wrap cord, twine or other flexible binding material around the package and to secure the ends of the binding material together to form a stable bale.
Commonly the baling chamber has at least one movable wall portion whereof the position can be changed to vary the outlet section of the baling chamber. An enlarged outlet section reduces the forces needed to advance the bales in the baler and hence diminishes the density of the newly formed bales. Otherwise, a reduced section substantially increases the friction forces of the bale along the chamber and thereby raises the compression forces exerted by the plunger on the crop material. Accordingly the density of the new bales will increase.
It is known in the art to adjust the position of the movable wall in accordance with a signal of a load sensor in order to produce bales having a predetermined density. Typically the position of the rear portion of the baling chamber is controlled by a hydraulic cylinder in response to a sensor monitoring the forces exerted by the plunger. EP-A-0 379 230 shows a baler having a plunger which is connected by a pair of wrist pins to a pair of connecting rods, which in turn are linked to a pair of rotating cranks on a gearbox. The wrist pins are equipped with strain gauges for monitoring the forces applied by the plunger. As the full compression force on the plunger is transferred upon the wrist pins, they have to be of a sturdy design. However, some deformation must be allowed to make the sensor furnish discernible signals which can be used in adjusting the outlet area of the baling chamber.
Other sensor embodiments are illustrated in U.S. Pat. No. 5,123,338. According to one embodiment, a substantially mechanical sensor reacts to the forces applied by the plunger on the connecting rod. The result is a mere ON-OFF signal which provides no intermediate data on the actual plunger load. Such output cannot be used for a refined adjustment of the bale density. The compression level achieved by this control system has to be adjusted by mechanical means on the baler itself.
Another embodiment in the same document shows a load sensor arranged between the gearbox driving the plunger and the main frame of the baler. The reaction forces of the plunger on the gearbox are measured by strain gauges affixed to an arm which is interconnecting the top of the gearbox and the frame. The output signal is variable such that it can be used for setting a wide range of densities.
Another sensor embodiment is described in EP1066748. In this embodiment, the force exerted by the plunger on the package of crop material is measured. The operator is informed of the measurement results, and can choose to adjust the position of the movable wall sections in the baling chamber. Thereby, the relationship between the force needed to push the package further into the baling chamber and the density of the package is used to steer the actuators moving the movable wall parts. This way of steering enables to obtain a relatively accurate regulation of the density of the package.
Recently, high density balers are developed, which are provided to push the crop material into a package with a density that is higher than densities that are obtainable via currently available balers. Therefore, these high density balers are provided with more heavy machinery.
A problem arises when using conventional density control systems together with such more heavy machinery. In a normal working mode, a movable wall section has on one side an actuator pushing the wall section inward with a predetermined force, and on the other side a package of crop material providing backpressure to this force. In startup mode however, the backpressure is absent and the heavy machinery of a high-density baler proved to be powerful enough to damage the movable wall section. Also during use, the crop material could be unequally spread through the baling chamber so that the backpressure at the movable wall section is not enough to counter the force exerted by the actuator in an attempt to reach a predetermined density, so that the actuator damages the movable wall section. Existing density control systems do not provide a solution to this problem. A density control system according to the prior art can consequently not be used in a high density baler.